1. Field of the Invention
The present invention relates to an exhaust gas purification apparatus that selectively reduces and purifies nitrogen oxide (NOx) in exhaust gas, and relates to a method for thawing a liquid reducing agent or precursor thereof used in the exhaust gas purification apparatus.
2. Description of Related Art
As an exhaust gas purifying system that purifies NOx contained in exhaust gas from an engine, an exhaust gas purification apparatus as disclosed in Japanese Patent Application Laid-open Publication No. 2012-13060 has been proposed. In this exhaust gas purification apparatus, a urea aqueous solution is injected, in accordance with an engine operation state, on an exhaust gas upstream side of a selective catalytic reduction (SCR) converter disposed in an exhaust pipe of the engine, and, using ammonia produced by hydrolysis, NOx is selectively subjected to a reduction reaction by the SCR converter and thus purified into harmless components.
In a cold area or the like, the temperature of outside air in winter decreases to or below the freezing point of a urea aqueous solution, and a urea aqueous solution stored in a tank gradually freezes, with the result that the urea aqueous solution cannot be injected on an exhaust gas upstream side of the SCR converter. For this reason, a conventional technique proposed in Patent Document 1 employs a configuration in which the urea aqueous solution frozen in the tank is thawed using engine coolant.
However, for example, right after cold start or the like, the temperature of engine coolant is low. In particular, in a small-displacement engine that generates less heat, increase in the temperature of the engine coolant is gradual, making it difficult to thaw the urea aqueous solution in the tank in a short time.